ChemComm
Open Access Article. Published on 04 June 2014. Downloaded on 19/06/2017 04:16:18.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
COMMUNICATION
Cite this: Chem. Commun., 2014,
50, 8215
Received 10th March 2014,
Accepted 30th May 2014
View Article Online
View Journal | View Issue
Surfactant-free CO2-based microemulsion-like
systems†
Robert F. Hankel,‡a Paula E. Rojas,‡bc Mary Cano-Sarabia,bc Santi Sala,bc
Jaume Veciana,bc Andreas Braeuer*a and Nora Ventosa*bc
DOI: 10.1039/c4cc01804d
www.rsc.org/chemcomm
The presence of water-rich and water-lean nanodomains in a transparent, pressurized ‘‘water–acetone–CO2’’ mixture was revealed by
Raman spectroscopy. This nano-structured liquid can be classified as
a surfactant-free microemulsion-like system and has the capacity to
dissolve hydrophobic compounds, such as ibuprofen, in the presence
of large amounts of water. This finding opens new opportunities in
the fields of confined reactions and material templating.
Microemulsions were introduced in 1943 by Hoar and Schulman,1 who titrated a milky emulsion with hexanol yielding a
transparent one. Since that time microemulsions have attracted
increasing attention because of their numerous product applications. Microemulsion-like systems are macroscopically isotropic
mixtures of at least one hydrophilic, one hydrophobic and one
amphiphile compound. They differ from conventional emulsions
in their thermodynamic stability and nanostructure.2,3 Since
microemulsions contain a polar as well as a nonpolar compound,
they are capable of solubilizing a wide spectrum of solutes and
thus are a versatile reaction medium with applications ranging
from nanoparticle templating to preparative organic chemistry.
Because of its spatial confinement, the dispersed phase of the
microemulsions can be used as nano-reactors with unique interfacial properties. Microemulsions feature a straightforward onestep formation procedure, without elaborate synthetic stages.4
‘‘Green Chemistry’’ ambitions motivate attempts to produce
a
Lehrstuhl für Technische Thermodynamik (LTT) and Erlangen Graduate School
in Advanced Optical Technologies (SAOT) Friedrich-Alexander Universitaet
Erlangen-Nuernberg, Paul-Gordan-Strasse 6, 91052 Erlangen, Germany.
E-mail: [email protected]; Tel: +49 9131 8525853
b
Department of Molecular Nanoscience and Organic Materials, Institut de Ciència
de Materials de Barcelona (ICMAB- CSIC), Campus la Universitat Autonoma
Barcelona (UAB), 08193 Bellaterra, Spain. E-mail: [email protected];
Tel: +34 935801853
c
CIBER-BBN: Campus Rı́o Ebro - Edificio I+D Bloque 5, 1a planta C/ Poeta Mariano
Esquillor s/n, 50018 Zaragoza, Spain
† Electronic supplementary information (ESI) available: Materials, ibuprofen
solubilization experiments, experimental procedures for Raman spectroscopy
acquisition and data treatment. See DOI: 10.1039/c4cc01804d
‡ These authors contributed equally.
This journal is © The Royal Society of Chemistry 2014
microemulsion with a reduced amount of surfactants or – in
the best case – without any surfactant. Even though surfactantfree microemulsions have been known since the late 1970s, only
scarce papers deal with this topic.5–8
Here we report for the first time – to the best of the authors’
knowledge – a surfactant-free pressurized microemulsion-like
system composed of water, acetone and CO2 showing the
capability of solubilizing ibuprofen, a hydro- and CO2-phobic
compound, which is soluble in CO2-expanded acetone. CO2based microemulsions containing surfactants are known,9–13
but not surfactant-free ones.5,14
Unlike water, CO2 is nonpolar and features weak van der Waals
forces, even if compressed. Thus, water and CO2 represent
extremes among the solvents. Therefore, pressurized microemulsions containing water as well as compressed CO2 have the
potential to act as universal solvents for a wide variety of solutes.10
The phase behavior of ternary mixtures of water, acetone and
CO2 at high pressure has been the subject of many studies revealing
a high pressure fluid–fluid phase equilibrium.15,16 Therefore,
according to a phenomenon called ‘‘salting out with a supercritical
fluid’’ which was first described by Elgin and Weinstock in the late
1950s, initially homogeneous mixtures of acetone and water can be
split in two fluid phases by pressurization with CO2.17
We have used Raman spectroscopy to gain an insight at the
molecular level of the effect of the addition of compressed CO2
over a mixture of water and acetone, before the system separates
in two liquid phases. Vibrational spectroscopy allows probing the
local environment of a molecule.18 Therefore, Raman spectroscopy provided insights into the state of aggregation of water
molecules in the ternary systems at high pressure. The existence
of the microemulsion-like system is proven from the Raman
spectrum of the OH stretch vibration of the water molecules,
which gives insights into the development of the hydrogen
bonding on a molecular level. The broad Raman spectrum of
the OH stretch vibration of liquid water covers the wavenumber
range between 3050 and 3850 cm 1 and can be deconvoluted into
several single peaks, representing different states of hydrogen
bonding between the water molecules.19 While the number and
Chem. Commun., 2014, 50, 8215--8218 | 8215
View Article Online
Open Access Article. Published on 04 June 2014. Downloaded on 19/06/2017 04:16:18.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Communication
the central wavenumber positions of the single peaks are still
debated, there is a general consensus in the literature that
the low wavenumber Raman signals (here between 3050 and
3450 cm 1) can be assigned to the hydrogen bonded water
molecules and that the high wavenumber Raman signals (here
between 3450 and 3850 cm 1) can be assigned to least bonded
water molecules. We set the wavenumber position of the
temperature-isosbestic point20 of the Raman OH stretch vibration
of pure liquid water at 3450 cm 1, the border between bonded
and least (here called ‘‘non-bonded’’) Raman signals. Along
this communication the Raman signals assignable to the nonhydrogen-bonded and to the hydrogen-bonded water molecules
are referred to as Inb and Ib, respectively. Furthermore, we use
ROH as the ratio Ib/Inb. A detailed description of the determination
of ROH from the acquired raw Raman spectra is given in the ESI.†
Temperature-,21 composition-22 or density23 variations in the
systems influence the development of the hydrogen bonding,
and are therefore observable in the OH stretch vibration Raman
spectrum of water and consequently influence ROH.
In order to demonstrate the existence of a microemulsion-like
system composed of acetone, water and CO2 we carried out two
Raman experiments. We diluted an equimolar mixture of acetone and water at 10 MPa and 308 K isobaric and isothermal,
once by the addition of acetone and once by the addition of CO2.
During the addition of acetone or CO2 we kept the pressure
constant by increasing the volume of the pressurized system.
For both experiments we computed the ratio ROH from the OH
stretch vibration Raman signal of water to monitor the evolution
of the development of the hydrogen bonding between the water
molecules. It should be mentioned here that at 10 MPa and
308 K, CO2 shows low solubility in water, whereas it is completely
miscible with acetone.24 Fig. 1 shows the two mixing paths in a
triangle phase composition diagram when either acetone or CO2
are added to the equimolar acetone–water mixture. Furthermore
the evolutions of ROH are given as a function of the water molar
fraction xH2O. It should be mentioned here that a decrease of the
water mole fraction goes in hand with an increase of the volume
for all the reported experiments. If acetone is added to dilute the
equimolar mixture, ROH decreases continuously as a function of
the decreasing molar fraction xH2O within the analyzed range.
This indicates a continuous decrease of the number of hydrogen
bonded water molecules (proportional to Ib) relative to the
number of non-bonded water molecules (proportional to Inb).
The dilution by the addition of acetone increases the mean
distance between the water molecules and therefore weakens the
hydrogen bonds. This effect, which is based on the consideration of the mean distance between the water molecules, is
referred to as dilution effect.
If CO2 is added to dilute the initially equimolar acetone–
water mixture, ROH first decreases (0.50 4 xH2O 4 0.45) and
later increases (0.45 4 xH2O 4 0.41) with decreasing molar
fraction of water (xH2O). As observed in Fig. 1, the initial ROH
decrease is not as strong as when adding acetone. Therefore,
another effect occurs in the system that counteracts the dilution effect. From xH2O = 0.45, ROH increases with decreasing
xH2O, indicating that this second effect is now stronger. In the
8216 | Chem. Commun., 2014, 50, 8215--8218
ChemComm
Fig. 1 (a) Evolution of the ROH ratio as a function of xH2O of the mixtures,
during the progressive addition of CO2 (black diamonds) and acetone (red
dots), at 10 MPa and 308 K, over an initial equimolar mixture of acetone
and water; (b) a zoom of the ROH ratio evolution during the progressive
addition of CO2; (c) the composition evolution of the system upon
progressive addition of CO2 (black diamonds) and acetone (red dots)
represented in a ternary diagram.
further progress of this communication this second effect is
called clustering effect. Scheme 1 shows the dilution effect for
the addition of acetone and the clustering effect for the addition of CO2 to the initially equimolar acetone–water mixture.
With respect to the evolution of ROH represented as a function
of xH2O in Fig. 1, the addition of CO2 first increases the effective
distance between the water molecules, which is represented
by a decrease of ROH for water molar fractions above 0.45. For
molar fractions below 0.45 the addition of more CO2 has to
promote the formation of water clusters, in which the effective
distance between the water molecules decreases, while at the
same time the mean distance between the water molecules in
the system still increases with further dilution. In other words,
the system is still homogeneous on a macroscopic scale, but
inhomogeneous on a molecular level.
If the water molecules cluster to water-rich regions, as shown
in Scheme 1, other water-lean nanoscopic regions should exist
with higher acetone and CO2 content relative to the bulk. These
nanodomains of different compositions are supposed to be
organized at the nanoscopic scale forming a macroscopically
homogeneous single phase, which shows the physical characteristics of a surfactant-free microemulsion-like system.
In order to further support the existence of CO2-expanded
acetone nanodomains in this nanostructured and macroscopically homogeneous microemulsion-like system, we studied
the power of the microemulsion to dissolve a non-water and
This journal is © The Royal Society of Chemistry 2014
View Article Online
Open Access Article. Published on 04 June 2014. Downloaded on 19/06/2017 04:16:18.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
ChemComm
Scheme 1 Difference of adding acetone or CO2 to the initial mixture.
(a) Mixture of water and acetone; (b) addition of CO2 first dilutes the
system and increases the mean distance between the water molecules;
(c) formation of water-rich and water-lean regions is initiated by adding
more CO2. (d) and (e) Addition of acetone dilutes the system and increases
the mean and the effective distance between the water molecules.
non-CO2 soluble compound. Then we compared the results
with the solvation capacity of a CO2-free equimolar acetone–
water mixture. We chose ibuprofen, which is a widely used antiinflammatory substance with a poor solubility in water and CO2
(0.18 10 5 mol mol 1 ref. 25 and 0.13 10 2 mol mol 1
ref. 26 respectively), but which has a high solubility in CO2expanded acetone at 10 MPa and 308 K.27
As shown in Fig. 2, the addition of CO2 over an equimolar
water–acetone mixture saturated with ibuprofen, where a solid
phase and a liquid phase coexist, at 10 MPa and 308 K causes
unexpectedly the solubilization of the precipitated CO2-phobic
drug. This unforeseen behavior is clearly explained if the addition
of CO2 causes a structuring at the nanoscale in water rich regions
and in water lean, CO2-expanded acetone domains, where the
ibuprofen is dissolved. Therefore the ibuprofen experiments as
Fig. 2 (a) Initial equimolar acetone–water mixture saturated with ibuprofen at 10 MPa and 308 K. (b) Complete dissolution of ibuprofen in the
microemulsions after the isothermal and isobaric addition of CO2.
This journal is © The Royal Society of Chemistry 2014
Communication
well as the Raman experiments point towards the existence of a
microemulsion-like system.
In summary, in this communication we have demonstrated
that surfactant-free pressurized microemulsion-like systems
can be formed by adding CO2 to mixtures of water and an
organic solvent, such as acetone, with high mutual miscibility.
The strong affinity of CO2 for acetone and its low solubility in
water24 cause the rupture of the homogeneity of the initial
water–acetone mixture and the structuration of the system at
the nanoscale. Raman spectroscopy is a useful technique to
characterize the microenvironment of water molecules in this
system and was used to prove the existence of water-rich and
water-lean nanodomains in the macroscopically transparent
liquid phase. These results are promising since microemulsions with compressed CO2 are an effective way to enhance the
solubility of solutes. Surfactant-free CO2-based microemulsions
enable more environmentally friendly process routes. Future
work will include more fundamental studies of the properties
of these systems and their use as templates for confined
crystallizations.
This work was supported by project POMAS (Grant
CTQ2010-019501) and the funding of the Erlangen Graduate
School in Advanced Optical Technologies (SAOT) by the
German Research Foundation (DFG) in the framework of the
German excellence initiative. We also acknowledge financial
support from Instituto de Salud Carlos III, through ‘‘Acciones
CIBER’’, with assistance from the European Regional Development Fund and the European Project BERENICE Project
(305937-2). Paula Elena Rojas thanks Consejo Superior de
Investigaciones Cientı́ficas (CSIC) for her PhD bursary and
she is enrolled in the PhD program of UAB. Robert Hankel
and Andreas Braeuer acknowledge Professors Leipertz and Will
for providing access to the experimental Raman equipment.
Notes and references
1 T. P. Hoar and J. H. Schulman, Nature, 1943, 152, 102.
2 Microemulsions: Background, New Concepts, Applications, Perspectives,
ed. C. Stubenrauch, Wiley, 2009.
3 D. G. Shchukin and G. B. Sukhorukov, Adv. Mater., 2004, 16, 671.
4 R. Wang, W. Leng, Y. Gao and L. Yu, RSC Adv., 2014, 4, 14055.
5 M. L. Klossek, D. Touraud, T. Zemb and W. Kunz, ChemPhysChem,
2012, 13, 4116.
6 B. A. Keiser, D. Varie, R. E. Barden and S. L. Holt, J. Phys. Chem.,
1979, 83, 1276.
7 B. A. Keiser and S. L. Holt, Inorg. Chem., 1982, 21, 2323.
8 N. F. Borys, S. L. Holt and R. E. Barden, J. Colloid Interface Sci., 1979,
71, 526.
9 K. P. Johnston, K. L. Harrison, M. J. Clarke, S. M. Howdle,
M. P. Heitz, F. V. Bright, C. Carlier and T. W. Randolph, Science,
1996, 271, 624.
10 C. T. Lee, W. Ryoo, P. G. Smith, J. Arellano, D. R. Mitchell,
R. J. Lagow, S. E. Webber and K. P. Johnston, J. Am. Chem. Soc.,
2003, 125, 3181.
11 J. Eastoe, C. Yan and A. Mohamed, Curr. Opin. Colloid Interface Sci.,
2012, 17, 266.
12 M. Klostermann, T. Foster, R. Schweins, P. Lindner, O. Glatter,
R. Strey and T. Sottmann, Phys. Chem. Chem. Phys., 2011, 13, 20289.
13 J. Zhang and B. Han, J. Supercrit. Fluids, 2009, 47, 531–536.
14 G. D. Smith, C. E. Donelan and R. E. Barden, J. Colloid Interface Sci.,
1977, 60, 488–496.
15 P. Traub and K. Stephan, Chem. Eng. Sci., 1990, 45, 751–758.
Chem. Commun., 2014, 50, 8215--8218 | 8217
View Article Online
Communication
22 O. S. Knauer, M. C. Lang, A. Braeuer and A. Leipertz, J. Raman
Spectrosc., 2011, 42, 195–200.
23 T. Kawamoto, S. Ochiai and H. Kagi, J. Chem. Phys., 2004, 120,
5867–5870.
24 P. G. Jessop and B. Subramaniam, Chem. Rev., 2007, 107, 2666–2694.
25 J. Manrique and F. Martinez, Lat. Am. J. Pharm., 2007, 26, 344–354.
26 D. Suleiman, L. A. Estévez, J. C. Pulido, J. E. Garcı́a and C. Mojica,
J. Chem. Eng. Data, 2005, 50, 1234–1241.
27 M. Muntó, N. Ventosa, S. Sala and J. Veciana, J. Supercrit. Fluids,
2008, 47, 147–153.
Open Access Article. Published on 04 June 2014. Downloaded on 19/06/2017 04:16:18.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
16 R. Adami, J. J. Schuster, S. Liparoti, E. Reverchon, A. Leipertz and
A. Braeuer, Fluid Phase Equilib., 2013, 360, 265–273.
17 J. C. Elgin and J. J. Weinstock, J. Chem. Eng. Data, 1959, 4, 3–12.
18 B. Renault, E. Cloutet, H. Cramail, T. Tassaing and M. Besnard,
J. Phys. Chem. A, 2007, 111, 4181–4187.
19 D. M. Carey and G. M. Korenowski, J. Chem. Phys., 1998, 108, 2669–2675.
20 G. E. Walrafen, M. S. Hokmabadi and W. H. Yang, J. Chem. Phys.,
1986, 85, 6964–6969.
21 M. Becucci, S. Cavalieri, R. Eramo, L. Fini and M. Materazzi, Appl.
Opt., 1999, 38, 928–931.
ChemComm
8218 | Chem. Commun., 2014, 50, 8215--8218
This journal is © The Royal Society of Chemistry 2014